1. Field of the Invention
This invention relates to metal semiconductor field effect transistors (MESFETs) and more particularly to fabricating MESFETs with submicron gates.
2. Description of the Prior Art
There has been a considerable effort to extend the frequency of operation of submicron GaAs MESFET devices to mm wave regions. Submicron MESFET devices have been fabricated by electron beam lithography, angle evaporation, by under etching the gate metal with another metal acting as the mask, and by metal lift-off techniques using optical lithography.
In a publication by M. Hatzakis et al., entitled "Single-Step Optical Lift-Off Process," IBM J. Res. Develop., Vol. 24, No. 4, Pp. 452-460, July 1980, a process is described that allows the use of the lift-off metalization technique with ultraviolet exposure of a single layer of photo resist. The process consists of soaking the resist layer for a predetermined time either in chlorobenzene or other aromatic solvents such as toluene and benzene before or after exposure. After development, resist profiles with overhangs suitable for lift-off metalization were obtained.
In a publication by A. Fathimulla, one of the inventors herein, entitled "Single-Step Lift-Off Process Using Chlorobenzene Soak on AZ4000 Resists", J. Vac. Sci. Technol. B3(1), Pp. 25-27, Jan./Feb. 1985. A single-step lift-off process using chlorobenzene soak on AZ4000 resists was described. The process provided thick overhang with vertical sidewalls, which are essential for the successful lift-off. UV hardening of the photo resist, AZ4110, increased the thermal stability of the overhang at temperatures obtained during deposition of metals such as tungsten. FIG. 3(a) of the publication shows a typical cross section of the photo resist having an overhang at the upper edge surface of the resist.
In a publication by H. Hiraoka et al. entitled "High Temperature Flow Resistance of Micron Sized Images in AZ Resists," J. Electrochem. Soc., Vol. 128, No. 12, Pp. 2645-2647, December 1981, a simple UV curing process is described that renders micron sized images in AZ resists resistant to flow when heated to temperatures as high as 210.degree. C. The ultraviolet exposure, however, will only be effective when done without a post bake. It is a common practice to post bake resists in order to enhance the quality of the film.
In a publication by R. Allen, entitled "Deep U.V. Hardening of Positive Photoresist Patterns", J. Electrochem. Soc., Vol. 129, No. 6, Pp. 1379-1381, June 1982, the thermal deformation of positive photoresist patterns is significantly reduced during high temperature treatment by flood exposure of the photoresist with a low pressure mercury lamp containing deep UV at .lambda.=254 nm. The stability of the photoresist patterns during high temperature after deep UV exposure was found to be due to the formation of an outer skin of higher polymerization.